Question suggestion methods have been widely used in social chatbots [1]}, which are designed for building long-term emotional connections with users. These questions often make a user stay longer and more engaged in the conversation, allowing the chatbot to learn more information about the user (e.g., user preference). For example, the user study of the Sounding Board system [2]}, which won the first Alexa challenge, demonstrates that the conversation recommendation function is crucial in maintaining user's interest in conversing with the system. Thus, the function has been widely adopted by other chatbots in later Alexa challenges [3]}.
